The present invention relates to a light source device, and a focusing mirror device and optical fiber connection device using the feature of the light source device and, more particularly, to a light source device which is improved to realize high energy density.
In general, focusing mirror devices are used in many fields such as headlamps for vehicles, various lighting fixtures, optical devices, lamp heaters for industrial equipment, and the like. In a conventional mirror device, its reflection surface has a nearly semi-elliptic sectional shape, and the mirror device is commonly disposed on the rear side of a light source to reflect light emitted by the light source by the semi-elliptic reflection surface and to project the reflected light forward.
FIG. 17 shows a conventional focusing mirror device that concentrates light to one point. In FIG. 17, the section of a reflection surface 3A of a mirror has an elliptic shape, and a light source 2 is placed at one focal point F1 of this elliptic shape. Light emitted by the light source 2 is reflected by the reflection surface 3A, and the reflected light is focused to the other focal point F2 as a converging point.
FIG. 18 shows a conventional focusing mirror device that obtains collimated light. In FIG. 18, the section of a reflection surface 3B of a mirror has a parabolic shape, and a light source 2 is disposed at the position of a focal point F1 of the parabolic shape. Collimated light is obtained by reflecting light emitted by the light source 2 by the reflection surface 3B. Note that a convex lens 4B having a small lens diameter is disposed in front of the center of the light source 2 to use light emitted by-the front surface of the light source 2 as collimated light.
As shown in FIG. 17, in the conventional focusing mirror device that brings light to a focus at one point, most light components L1 emitted forward (the right side in FIG. 17) by the light source 2 cannot be used since they diverge without striking the reflection surface 3A, and cannot be focused.
The divergence angle .theta.1 of the focused light becomes considerably large (e.g., about 58.degree.). For this reason, when light is projected farther using a lens, a lens 4A (indicated by the one-dashed chain line) is disposed so that its focal point matches the focal point F2. However, since the divergence angle .theta.1 is large, the amount of light that becomes incident on the lens 4A is small, and only half the focused light can be used. More specifically, even when a lens is designed to have a short focal length, its minimum focal length is nearly equal to the lens diameter. Under the circumstance, even when the lens is effectively used up to its outermost periphery, a divergence angle .theta.1 of only about 25.degree. can be covered, and light components reflected beyond this angle cannot become incident on the lens 4A, resulting in losses.
In this case, a Fresnel lens which has a short focal length and large diameter may be used. However, since this lens has a complicated shape, and poor optical precision, its application range is limited.
On the other hand, the focusing mirror device that obtains collimated light, as shown in FIG. 18, is used for, e.g., drying a paint. However, as shown in FIG. 18, the amount of light has high density on the central portion side, and its density decreases toward the peripheral portion of the reflection surface 3B. Hence, the irradiation surface cannot be irradiated with an amount of light with a uniform density. Furthermore, neither the irradiation spot size nor irradiation light amount density can be controlled.
The conventional light source device cannot cope with a case wherein an energy density equivalent to or as high as that of a laser beam is required. This is because only some light components of those emitted by a lamp can be used, and light emitted by the lamp cannot be sufficiently collected even using another mirror since it has a large divergence angle.
FIG. 47 is an explanatory view showing the concept of obtaining light with high energy density by converging light beams emitted by a plurality of lamps. As shown in FIG. 47, a plurality of, for example, three, lamps 2L are disposed, and reflection mirrors 5 each having a parabolic sectional shape are arranged in correspondence with these lamps. These mirrors reflect light beams emitted by the lamps 2L located at the focal points of the corresponding parabolas to obtain collimated light 6. Furthermore, the collimated light 6 is focused to one point on a target object TO set at the focal point by reflecting it by a final reflection mirror 8 having a parabolic sectional shape.
In this case, the diffusion angle of incoming light on the final reflection mirror 8 is determined by the reflection mirrors 5 for converting light emitted by the lamps 2L into collimated light, and the size of the emission portion of each lamp 2L, and their positional relationship. However, in such example of the arrangement, even when the number of lamps 2L is increased to increase the energy amount, a spread of light on the focal point of the reflection mirror 8, i.e., on the target object TO, increases, and consequently, the energy density does not become sufficiently high. Note that the diffusion angle indicates an angular error of light produced depending on the size or the like of a light source.
This point will be explained in detail below. FIG. 48 is an enlarged view showing one of the lamps shown in FIG. 47. In the illustrated example, light components emitted forward within the direction range from 0.degree. to 50.degree. cannot become incident on the reflection mirror 5 and cannot be used. Also, the luminous flux density of the collimated light becomes lower toward the peripheral portion of the reflection mirror 5, and the size of the final reflection mirror 8 cannot be utilized. Light rays near the center have a higher density, but have a larger diffusion angle.
The diffusion angle will be explained below using a diagram depicting the ray tracing result. FIG. 49 shows the ray tracing result obtained by extracting only 90.degree. light components of those emitted by the entire surface of a light source lamp having a finite size, and FIG. 50 shows the ray tracing result obtained when a reflection mirror having a size twice that shown in FIG. 49 is used. Note that the diameter of the light source lamp 2L is set at 1 mm.
As can be seen from FIG. 49, a light beam emitted by the light source lamp 2L having a diameter of 1 mm in the 90.degree. direction is reflected by the reflection mirror 5, temporarily converges, and then diverges. After that, the light beam is reflected by the reflection mirror 8, and becomes incident on the target object TO. Of the light beam, only the central light ray reaches a focal point FT, and marginal light rays reach positions separated from the focal point FT according to the positions of their origin. Hence, energy cannot be sufficiently concentrated.
Such phenomenon of light divergence is called diffusion. The error from an angle with a mathematically normal path determined by the shape of the reflection mirror or the maximum value of light rays suffering angular errors is defined as a diffusion angle. In case of a parabolic mirror, collimated light serves as a reference, and the diffusion angle means an angle the light ray makes with the collimated light. In case of a mirror having an elliptic section, the mathematically normal path is a straight line which connects each point on the elliptic mirror and the focal point, and the diffusion angle means an angle each light ray makes with the one which reaches the focal point. Note that "normal" merely means that it is intended in design of the mirror.
In FIG. 50, the width of the reflection mirror 5 is twice as large as that of the reflection mirror shown in FIG. 49. In this case, the diffusion angle becomes small, and the divergence at the focal point F1 is halved. The reason why a light beam diverges and the diffusion angle becomes large is that the light source that emits light essentially has a finite size. This point will be explained below with reference to FIG. 51. FIG. 51 is a view for explaining the cause of an angular error of light emitted by a light source having a finite size. In FIG. 51, reference numeral 2 denotes a filament serving as a light source; 5, a reflection mirror having a parabolic section. For the sake of easy understanding, the filament illustrated has a large diameter.
In this case, light emitted in the 45.degree. direction behind the light source 2 will be taken as an example.
Light originating from a central point S1 of the light source 2 and emitted in the 45.degree. direction behind the light source toward a point M1 of the reflection mirror 5 is reflected along a mathematically normal optical path, and reaches a mathematically normal focal point. However, light originating from an end point S2 of the light source 2 in the same direction as that of the above light, and propagating toward a point M2 of the reflection mirror 5 is reflected in a direction different from the above light. This light has an angular error of an angle .theta.d from each optical path that reaches the mathematically normal focal point.
The angle ed takes different values depending on the angles of light emitted by the light source 2 toward the reflection mirror 5, but the angular error produced is proportional to the size of the light source 2 and is inversely proportional to the distance between the light source 2 and reflection mirror 5. In this manner, since the light source has a finite size, an angular error is produced thereby, and the diffusion angle increases. As a result, sufficiently high energy density cannot be obtained.
Furthermore, since the conventional light source device can only reflect and focus some of light components emitted by the light source, such mechanism also fails to obtain sufficiently high energy density.